The present invention relates to exhaust and intake manifolds for reciprocating internal combustion engines. More in particular, the present invention relates to unique exhaust and intake manifolds that produce a wide band of relatively high engine torque.
An intake manifold of an internal combustion engine conducts combustion air to the cylinders of the engine where it burns with fuel. In a fuel injected engine, the manifold conducts only air; in a carbureted engine, a manifold also conducts fuel with the air. In a carbureted engine, the carburetor typically mounts on a plenum of the manifold. The fuel and air mixture enters the plenum from the carburetor, and from the plenum travels to the cylinders through ducts called runners. The runners exit into inlet ports in the cylinder head of the engine. These ports lead to the cylinders through inlet valves.
Generally, a vacuum in each cylinder created by downward piston movement during an inlet stroke draws the fuel and air mixture into the cylinder. A supercharger or a turbocharger can augment this driving force.
The burning of the fuel-air mixture in the cylinders generates high pressure products of combustion that expand against the pistons during an expansion stroke to produce the engine's power. Exhaust valves from the cylinders open while the products of combustion are still at high pressure relative to atmospheric pressure. This residual pressure, called blow-down pressure, and the ascent of the pistons in the cylinders force the products of combustion from the cylinders into an exhaust manifold.
The dynamics of induction of fuel and air into an engine and the exhaust of products of combustion from an engine are very complicated, making generalizations difficult. Some of the factors affecting induction and exhaust include intake and exhaust valve timing, piston speed, gas inertia, gas friction, resonance, intercylinder interference, and manifold geometry.
The intake valve timing of today's internal combustion engines has an inlet valve starting to open while its companion exhaust valve is in its final stages of closing and before its piston reaches top dead center. An inlet valve closes several crank degrees after its piston reaches bottom dead center. On the exhaust side, the exhaust valve opens several degrees before bottom dead center on the expansion stroke and closes after top dead center and after the exhaust stroke. The timing of inlet and exhaust valve opening and closing accommodates the several crank degrees of engine revolution needed to get them open an effect amount and to effectively close and to accommodate gas inertia.
It is quite apparent that the more mixture inducted into a cylinder with each cycle, the more power an engine will have, the more efficient it will be. A measure of engine efficiency reflecting the amount of cylinder charge is "volumetric efficiency," which is the volume of air a cylinder actually receives divided by the volume swept by the piston. With no induction loss, if the air travels fast in the runners towards the end of the induction cycle, its inertia overcomes the pressure build up at this time and results in an additional amount of mixture charged into the cylinders and an increase in volumetric efficiency compared with a charge from slower air. More specifically, increasing the velocity of the mixture at relatively low engine speeds enhances torque. (At high engine speeds, induction losses can more than offset gains from gas inertia.)
Piston speed directly measures the pumping characteristics of an engine. The higher the piston speed, the more mixture the pistons induct into the engine in a given time period. Piston speed also generates pressure pulses that affect movement of the mixture in the intake manifold. As the piston descends, a negative pressure signal results and this signal travels upstream in the manifold. It is this negative pressure that produces induction. As the piston ascends, it produces a positive pressure signal that travels upstream from the manifold and opposes induction. The magnitude of the signals is a direct function of piston speed, which varies even at constant engine speed. The pressure signals travel at the speed of sound; the mixture travels much slower. The pressure signals can be used to enhance volumetric efficiency by resonance.
My U.S. Pat. No. 4,461,248 describes one way to use pressure signals in an intake manifold to enhance an engine's performance. This patent builds on the long recognized fact that pressure pulses traveling up and down runners can affect the flow of air through runners. As a positive pressure pulse, positive with respect to mean inlet manifold pressure, travels up a runner and reaches atmosphere, which may be in the plenum of the manifold, the air there moves with the disturbance creating a depressed pressure or a locally rarified zone. The resulting negative pressure travels down the runners, and it detrimentally affects the flow of gas in the runners by reducing the pressure differential between the runner and the cylinder. A negative pressure pulse traveling upstream produced by a descending piston reduces the pressure in the plenum: air rushes in to fill the low pressure zone, generating a positive pressure pulse that travels down the runner toward the cylinder.
If a positive pressure pulse arrives at the cylinder at the right time, say when the inlet valve is about to close, the pulse can add significant quantities of mixture to the cylinder to increase the power of the engine by increasing the volumetric efficiency of the engine. When the length of runners is adjusted to take advantage of this phenomenon, it is known as intake manifold tuning. The time it takes for a pressure pulse to travel up and back in a runner depends on runner length. Because the time of the pulse's arrival back at the inlet port must be close to the time the inlet port is about to close, the speed of the engine must coincide with the pulse travel time. Tuning, in short, works in only limited speed ranges. When a manifold is tuned, it is said to resonate in the engine speed range that the positive pressure pulse augments cylinder charges because the pulse adds to the pressure driving the charge into the cylinders.
My '248 patent divides up the runners of an intake manifold into sets, with each set being tuned to produce resonance at engine speed ranges. The patent also teaches that combustion air speed at resonance should be the same as the speed at engine speed maximum torque and to adjust runner cross-sectional area so that this happens. This produces a broader band of high torque in the engine than would be the case with all the runners producing resonating in the same speed range.
As inlet manifold runner tuning was known long before my '248 patent, so was exhaust manifold tuning. Exhaust manifold tuning reduces residual exhaust gas pressure in the cylinders in a selected engine speed range so that the inlet charge sees low resistance. The idea in exhaust manifold tuning is to get a rarified pressure pulse in the vicinity of the exhaust valve just as combustion air begins to enter the cylinder and combustion air into the cylinder to draw products of combustion from the cylinder. In the case of exhaust tuning, the exhaust gas products and not piston motion creates the pressure pulse because of the high value of pressure at the time the exhaust valve opens.
As was recognized in my '248 patent, particularly with engines with modest power outputs, it is important to have a broad band of relatively high torque so that engine performance over a range of engine speeds is good.
It is desirable to enhance the improved performance produced by my '248 patent by using exhaust gas dynamics.